Service providers are beginning to develop, offer, and deploy wireless communication devices referred to as machine-type communication device. A machine type communication (MTC) device differs from traditional human-to-human (H2H) communication devices because a MTC device typically involves communication between entities that do not necessarily need human interaction. For example, a MTC device can be wireless user equipment configured to gather measurement information and report this information to a central server at a particular time interval. MTC devices can be used in a wide variety of contexts such as remote meter reading for water and power companies, wireless burglar and/or fire alarm monitoring, weather monitoring, vehicle tracking, medical monitoring, and the like.
MTC devices have operational characteristics that differ markedly from the operational characteristics of conventional human-to-human (H2H) wireless communication devices. Conventional H2H communication usually requires allocating resources for substantially continuous duplex communication between users for intervals as long as several minutes or even hours. In contrast, MTC devices typically transmit relatively small amounts of information in bursts separated by relatively long and sometimes irregular intervals. For example, a MTC device for remotely reading a water meter may only transmit a burst of information indicating water usage on a monthly basis. For another example, a burglar alarm monitor may only transmit bursts of information when the alarm is triggered. Consequently, MTC devices are also typically significantly more delay tolerant than conventional H2H devices since voice communication requires delays of less than 100 ms or better. A MTC device that reads and reports water usage may be able to tolerate transmission delays of days or even weeks. Moreover, MTC devices are often fixed to particular locations and so the mobility of these devices may be significantly lower than the expected mobility of a H2H device.
The distribution of MTC devices is expected to be significantly different than the distribution of handheld wireless communication devices. Current generations (2G/3G) of wireless communication systems have been designed to accommodate capacities on the order of 100 users per cell based on expected densities of H2H devices. However, the number of MTC devices in each cell is expected to be at least an order of magnitude higher and each cell may have to support thousands of MTC devices. Randomly transmitted access signals from such a large number of MTC devices, such as access signals transmitted over a random access channel, will almost certainly lead to a very large number of collisions. Furthermore, transmissions from some kinds of MTC devices tend to be strongly correlated in time. For example, an office building may have a very large number of remotely-monitored fire alarms. Under normal conditions the fire alarms generate virtually no traffic except perhaps a periodic “I'm alive” pulse to verify they are operating. However, if a fire breaks out all of the alarms may begin to concurrently transmit large bursts of information. Correlated bursts of information from large numbers of MTC devices in a cell can generate overload conditions, congestion, and collisions between access signals.
One proposal for flattening the time distribution of access signals from machine type devices is to allow a central entity to schedule the access signals using a polling scheme. The polling based scheme requires a central entity in the network (such as the E-UTRAN) to page each device at a predetermined reporting time to determine whether the device has information to transmit. Although one-by-one paging of the devices by the E-UTRAN scheduler could avoid collisions, this approach introduces much signaling overhead, particularly over the forward link. The efficiency gains from flattening the access transmission distribution are not thought to justify the high cost in overhead and complexity introduced by this method.
An alternative proposal is to apply the conventional random access method with access barring mechanisms such as using random back-offs to resolve collisions between the random access signals (access probes). Although this approach can flatten the time distribution of the access signals, the overhead costs would be considerable. For example, a large number of access collisions may be generated if a large number of MTC devices send random access request signals at the same time. Backing off some of the request signals would flatten the distribution but may still lead to additional collisions between retransmissions when the number of requesting devices is large. The efficiency of the system is therefore reduced (and the reverse link signaling overhead increased) by using back-offs and retransmissions to resolve the collisions. The retransmissions may also introduce more delay of reports from the MTC devices and create more uncertainty on the actual reporting time.